Transcutaneous Electrical Nerve Stimulation (TENS) technology is a well known and accepted method of treating pain whereby a very low current electrical signal (e.g., 1 to 150 Hz, 30 to 250 micro second pulse width, and voltages typically less than 400 V DC) is passed through the skin with conductive electrodes. There are three likely theories as to the mechanism of action:                A. Gating Theory—electrical stimuli generated by a TENS device excites sensory nerve pathways to the point where they become chemically fatigued and cease to send pain signals to the brain.        B. Endorphin Theory—a physiological response to the irritation produced by electrical stimuli is pain induced endorphin release.        C. Physiological Change Theory—the pulsed signal of a TENS stimulation can cause localized muscle activity leading to increased circulation in the treated areas and removal of metabolic toxins.        
Traditional heat therapy may predate mankind as even animals are known to soak themselves in hot springs. Heat usually feels good because of the increased circulation it generates in affected tissue. This increased circulation relieves pain by improving tissue oxygenation thus removing metabolic toxins. The disadvantage of heat therapy is that it can aggravate an inflammatory response.
Cold therapy is less often chosen by individuals for pain treatment because of the discomfort of ice on the skin. However, it is widely recognized as a powerful treatment for many types of pain by physicians and therapists and is often used for acute injuries. This therapy has two known mechanisms of action:                A. Although initially uncomfortable, cold contact is a heat sink that acts as a powerful anti-inflammatory by rapidly drawing heat out of affected tissue.        B. Cold temperatures have an anesthetic and numbing effect on sensory nerves.        
The drawback to cold therapy is that prolonged application, and excessively reduced temperature can cause decreased blood supply producing shivering and muscle tension.
The combined effects of thermal therapy and TENS therapy are disclosed in U.S. Pat. Nos. 5,097,828, 5,314,423 and 5,336,255. U.S. Pat. No. 5,314,423 describes a pain alleviating tissue treatment assembly that uses a combination of a cold electrode and an alterable current source to reduce the temperature of a selected tissue area while simultaneously applying various selected reversing and/or non-reversing currents to the selected tissue. This patent teaches that electrode chilling allows nerve stimulation enhancement with significantly greater energy input while not increasing patient discomfort.
U.S. Pat. No. 5,336,255 teaches an electrical stimulation heat/cool pack pouch made from waterproof lined fabric connected by loop and hook devices (Velcro) to conductive fabric patch electrodes backed with a wire lead. This device has built in insulating air gaps (Velcro) between the electrodes and the heat sink/reservoir such that the heat transfer is minimized at the electrode sites.
U.S. Pat. No. 5,097,828 discloses a therapeutic device for heating or cooling the skin that includes a handle, a thermally conductive head secured to the handle, and a thermally conductive member secured to the front end of the head. A thermoelectric means heats or cools the thermally conductive member and the head includes a thermally conductive portion adjoining the thermoelectric means such that the head can function as a heat sink.
Both hot and cold packs can be uncomfortable and sometimes damaging to tissue depending on temperature, contact time of the thermal reservoir, and thermal conductivity of the containment material. In tests for the present invention, applied heat has been determined to be less effective with TENS therapy than applied cold with TENS therapy.
Drawbacks do exist with prior art. U.S. Pat. Nos. 5,314,423 and 5,097,828 teach actively chilled electrodes and contact devices, however, these methods are impractical for portable use as they require support devices such as fluid chillers in one case, and in the case of Peltier chilled electrodes, cumbersome power supplies and heat exchanger devices. In addition, the chilling attachments to the electrodes can be awkward for mobile TENS application. Further, with respect to U.S. Pat. No. 5,336,255, cloth on wire type TENS electrode patches are usually fragile. When they inadvertently separate from the skin, the electrical discharge from the reduced contact area can sting and even burn the skin. In addition, electrically conductive rubber patches (usually carbon filled silicone) often have high electrical resistance of about 20 kΩ through a 1 mm thickness.
Electrode patch adhesives used in the prior art also share drawbacks. First, electrode patch adhesives can pull hair on removal from the skin and these adhesives are sometimes difficult to clean. Second, as the skin sweats, the adhesives may lose some of their grip and develop a change in electrical conductivity related to fluid absorption in the adhesive. Finally, adhesives dry out causing changes in stiction and conductivity. TENS electrodes and adhesives are typically such poor thermal conductors, that if one tries to combine an ice bag placement over a set of electrode patches, the treated area will have warm spots at the electrode sites.